by Emil Zanchetta Ulsig

• This PhD thesis explores new frontiers in photonics, focusing on how light can be manipulated in innovative ways to achieve previously unreachable wavelengths. By leveraging advanced materials like GaAs, AlGaAs, and InGaP integrated on silicon, it opens up possibilities for breakthroughs in quantum communication and environmental sensing. The work not only demonstrates new techniques for light manipulation but also moves closer to real-world applications, with potential to scale from the lab to transformative technologies in secure communications and precision gas sensing. These contributions highlight the exciting potential of integrated photonics to drive impactful advancements across various industries.
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by Esben Madsen

• This MSc thesis explores cutting-edge methods to stabilize laser signals through optical phase locking, using nonlinear frequency conversion on a photonic integrated circuit. By leveraging the fixed phase relation in three-wave mixing, the project successfully stabilizes a frequency-converted signal to a reference laser, marking a key step in advancing laser control technologies. With custom-built hardware, the research also reveals opportunities for enhancing signal modulation and conversion efficiency. This work offers fresh insights into the integration of nonlinear optics with scalable photonic systems, paving the way for more reliable and efficient optical technologies.
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by Emre Aslan

• This MSc thesis explores innovative enhancements to the self-homodyne coherent receiver (SHCR) method for measuring laser frequency noise, focusing on improving precision and reducing internal noise. By incorporating differential receiver amplifiers and employing a cross-correlation technique, the research significantly lowers the noise floor, allowing for accurate frequency noise measurements of lasers with intrinsic linewidths as narrow as 3.2 kHz. The work presents a detailed analysis of the experimental setup, optimizing key components such as fiber delay lengths and signal processing techniques. Ultimately, the study highlights SHCR as a cost-effective and powerful alternative to commercial devices, with potential applications in quantum communication and environmental sensing.
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by Maria Fernanda Alemán Magdaleno

• This MSc thesis explores the generation of mid-infrared light through difference-frequency generation (DFG) in a gallium arsenide (GaAs) suspended waveguide. Targeting a wavelength of 8.46 μm, the study focuses on designing a waveguide that leverages GaAs's high refractive index contrast with air, promising advancements over existing technologies that are often expensive or impractical. The simulation results highlight optimal waveguide dimensions, demonstrating a maximum output power close to 1 mW, influenced by factors like buffer slab thickness, height, and width of the waveguide. The thesis underscores the potential of GaAs waveguides for applications in industrial sensing, environmental monitoring, telecommunications, and quantum information. It suggests further research and experimental validation to harness the full potential of this promising technology.
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by Maria Paula Montes Bejarano

• This MSc thesis presents groundbreaking advancements in photonic technology through the exploration of dual-wavelength emission in a monolithic InP photonic integrated circuit laser, achieving remarkable linewidth narrowing via external optical feedback. The study successfully characterizes dual emissions in the telecom C-band at 1544 nm and 1549 nm. Introducing optical feedback dramatically enhances performance, reducing linewidth by up to two orders of magnitude to an impressive 20 kHz for both wavelengths. This innovation, coupled with the laser's compact form factor and streamlined manufacturing process, highlights its potential for transformative applications in telecommunications and medical fields. Additionally, the 5 nm wavelength separation paves the way for exciting possibilities in Terahertz signal generation using the optical heterodyne technique. This thesis not only underscores the laser's current capabilities but also sets the stage for future research into theoretical models and feedback polarization effects, solidifying its role as a game-changer in photonic technology.
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by Lucas Christesen Ahler

• This MSc thesis showcases the potential of InGaP-on-insulator photonic integrated circuits (PICs) in advancing χ(2) frequency conversion technologies. The research achieves efficient second-harmonic generation (SHG) with an impressive internal normalized conversion efficiency higher than 50 %/W. Ongoing collaborations and future research directions promise further enhancements, positioning InGaP-on-insulator PICs as a leading platform in integrated photonics applications.
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by Arseniy Alexander Frost

• This MSc thesis explores the feasibility of using Raman and absorption spectroscopy to detect and analyze D-glucose concentrations. It introduces a novel approach utilizing nonlinear optical chips for generating relevant wavelengths. The research achieves significant results, particularly in Raman spectroscopy, demonstrating a mean absolute relative difference of 6.76 percent using a Wasatch spectrometer. Despite challenges like interference from demineralized water in mid-infrared absorption, the study highlights the potential of nonlinear optical chips for sum and difference frequency generation. Future recommendations aim to refine sample preparation and enhance background subtraction, paving the way for advanced non-invasive glucose monitoring techniques.
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by Kevin Bach Gravesen

• This MSc thesis presents groundbreaking advancements in nonlinear integrated photonics, blending rigorous theoretical frameworks with practical experiments. The theoretical cornerstone is a pioneering formalism derived from Maxwell’s equations, tailored for nanophotonic waveguides and heterogeneous structures. The study yields analytical solutions, including the discovery of bright temporal solitons and comprehensive insights into the Kerr effect. Practical achievements include successful coupling of a high-power blue laser diode to a SiN nanowaveguide, showcasing potential for on-chip power enhancement despite initial challenges. Looking forward, the thesis suggests exciting avenues for further research, such as validating theoretical predictions through experimental super-continuum generation and optimizing coupling strategies for improved device performance.
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by Frederik Emil Philip & Frederik Ruben Bruun Sørensen

• This MSc thesis investigates the potential of amorphous alumina waveguides for guiding light and explores their application in Third Harmonic Generation (THG) of far-UVC light. The research demonstrates that these waveguides effectively guide blue light with low propagation losses but face challenges in confining red light without top cladding. Simulation results suggest solutions like increasing waveguide height or burying waveguides in silica to improve light confinement. A novel toolbox for measuring propagation losses using a commercial camera is developed, capable of analyzing complex waveguide structures such as spirals. Future work includes verifying simulation findings experimentally, exploring buried waveguides, and conducting wavelength sweeps to further characterize blue light propagation. This thesis concludes by emphasizing the potential for THG applications with tunable lasers and underscores the toolbox's versatility in advancing optical waveguide research.
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by Maria Paula Montes Bejarano and Emre Aslan

• This project delves into the precise characterization of laser frequency noise, focusing on two single-frequency diode lasers operating in the telecommunication C-band. Through meticulous measurement using both direct frequency noise measurements and the delayed-self heterodyne methods, the report reveals intriguing insights into their noise profiles. These include a dominant 1/f noise pattern, alongside distinct dither tones. These findings underscore the importance of precise measurement techniques in advancing laser metrology, promising advancements in high-stability applications within telecommunications and beyond.
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by Lucas Christesen Ahler

• This project delves into the innovative use of indium gallium phosphide (InGaP) microring resonators for second-harmonic generation (SHG), revealing their potential to revolutionize photonic integrated circuits (PICs). By leveraging InGaP's high nonlinear susceptibility and low loss at C-band wavelengths, the study showcases two microring designs optimized for quasi-phase matching and resonance, achieving remarkable SHG efficiency. Through sophisticated simulations and meticulous analysis, the research highlights how these designs can surpass current experimental efficiencies, paving the way for advanced applications in quantum communication and nonlinear optics. This groundbreaking work underscores the transformative potential of InGaP microrings in future photonic technologies.
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by Esben Rueskov Madsen

• This project aimed to harness 222nm far-UVC light through Second Harmonic Generation (SHG), offering a cost-effective solution compared to current methods. The focus was on modulating the pump laser diode's intensity to optimize the SHG chip's output power, which should ideally scale with the square of the input power. Successful modulation up to 1 MHz was achieved, demonstrating potential for efficient light modulation. The study highlighted the benefits of 222nm light for safe disinfection and explored its application in secure LiFi communication, leveraging its interference-free nature. Overall, the research advanced methods for integrating modulated light sources into practical applications, paving the way for future developments in both disinfection and communication technologies.
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by Mikkel Torrild Hansen

• This MSc thesis represents a pioneering effort in advancing photonics technology through the design and realization of a double-ridge waveguide platform in X-cut thin-film lithium niobate (TFLNOI). The research achieves remarkable success in achieving perfect type-0 phase matching for second harmonic generation (SHG), demonstrating impressive conversion efficiencies over a 6 cm interaction length. Despite encountered challenges in fabrication, innovative solutions such as the HOMTracker algorithm were developed to ensure robust performance under varying conditions. Looking forward, the platform shows promising potential for applications beyond SHG, including non-degenerate sum frequency generation (SFG) and spontaneous parametric down-conversion (SPDC). This work not only underscores the scalability of TFLNOI-based photonics devices but also sets a solid foundation for future advancements in integrated quantum photonics.
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by Mónica Far Brusatori

• This PhD thesis marks a transformative journey through the frontier of semiconductor laser technology, focused on overcoming the challenges posed by external optical feedback (EOF). From foundational theories to innovative design approaches, the thesis proposes and substantiates a novel strategy where EOF is managed bidirectionally within the laser cavity. Theoretical advancements predict and experimental validations demonstrate unprecedented stability and performance enhancements, including substantial linewidth reductions under varying feedback conditions. These findings not only validate the robustness of the proposed design but also underscore its potential to revolutionize future semiconductor laser applications, offering compact, high-performance solutions resistant to external disturbances.
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by Simon Thorndahl Thomsen

• The MSc thesis investigates the delayed self-heterodyne (DSH) method for measuring laser frequency noise using delays shorter than the intrinsic coherence length of lasers. Theoretical analysis confirms its efficacy in directly assessing frequency noise, while meticulous experimental characterization ensures minimal interference from instrument and laser noise sources. Practical application on 1550 nm external cavity diode lasers with kHz linewidths demonstrates that the method effectively reproduces frequency noise compared to commercial analyzers with 2-meter delays, yet shows limitations with longer delays due to accumulated phase noise. The study challenges the reliance on intrinsic linewidth as a sole measure of frequency noise, highlighting instead the method's robustness for lasers dominated by spontaneous emission noise. Future research aims to extend this methodology to different laser types like DFB and fiber lasers, exploring their unique noise characteristics and pushing the accuracy limits of the DSH method beyond the current 4.5 kHz linewidth threshold.
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by Pedro Henrique Godoy

• This MSc thesis explores the generation of light at 2325 nm using difference-frequency generation (DFG) in an AlGaAs-on-insulator waveguide. It investigates the theoretical framework and simulations underlying the DFG process, focusing on achieving perfect phase matching and predicting conversion efficiencies based on waveguide parameters, pump/idler power, and propagation losses. Applications include the potential integration with quantum dot single photon sources, enabling conversion to the telecom range, and the use of DFG for creating offset-free frequency combs essential for precision clockwork applications. The thesis discusses limitations, such as the need for experimental validation through chip fabrication and the exploration of temperature sensitivity in waveguide materials to enhance tunability. Ultimately, while not replacing all existing laser sources, the DFG-generated source shows promise for advancing mid-infrared photonics in sensing and quantum communication technologies.
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by Sebastian Folsach Thyge Hansen

• This MSc thesis presents groundbreaking advancements in enhancing communication links between the nacelle and the hub in wind turbines using the Fiber Optic Rotary Joint (FORJ). The research reveals that the FORJ maintains a broad transmission window with minimal signal loss, effectively integrating with various communication channels (850 nm MM, 1270/1330 nm SM, and 1470 nm SM). This discovery promises significant improvements in communication reliability. Despite some challenges with simultaneous channel use, the thesis outlines exciting future solutions, such as employing Array Waveguide Grating (AWG), exploring additional CWDM channels, optimizing rotational speed, using alternative splitters, and leveraging tunable laser modulation. These innovative strategies have the potential to revolutionize the system's efficiency and reliability, paving the way for robust two-way communication in wind turbine technology.
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by János András Németh

• This MSc thesis investigates the Self-Homodyne Coherent Receiver (SHCR) method for characterizing frequency noise (FN) in lasers. The study explores the method's dependency on fiber delay lengths and proposes an approximation to convert phase noise (PN) measurements into FN power spectral density (PSD) across different delays. Two lasers with varying noise characteristics are comprehensively analyzed and compared with commercial Frequency Noise Analyzers (FNAs). Results demonstrate that SHCR can accurately reproduce reference spectra when an appropriate delay is chosen, typically aligning with the laser's coherence length. The thesis identifies limitations, including the need for further validation across diverse laser types and suggests enhancements for measurement setup and DSP algorithms to improve accuracy. Despite these challenges, SHCR emerges as a cost-effective alternative to commercial instruments for FN characterization, warranting future research to refine its application and broaden its utility in laser technology and research contexts.
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by Niklas Hedegaard Arent

• This MSc thesis investigates methods for characterizing frequency noise (FN) power spectral density (PSD) in lasers and their implications on laser performance, particularly in terms of 3-dB linewidth. The study employs state-of-the-art commercial instruments to analyze FN PSDs across a spectrum of lasers, ranging from high-quality fiber lasers to outdated tunable lasers. Key findings include identifying distinct patterns in FN PSDs, such as the impact of operational modes and current sources on FN characteristics. The thesis also explores the delayed self-heterodyne (DSH) setup as a validation tool for narrow-linewidth lasers, revealing varied Gaussian and Lorentzian spread behaviors among different laser types. Furthermore, a Bayesian framework using extended and unscented Kalman filters demonstrates promising results in filtering noise from signals, highlighting its potential application in enhancing FN measurements' sensitivity and accuracy. Future prospects involve refining analytical models for DSH traces affected by 1/ f noise and implementing advanced filtering techniques like the cubature Kalman filter to further improve measurement precision in noisy environments.
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